What you maybe experiencing is the Auto Power Off features "kryptonite" - if you powered On and did not edit anything - power Off will wait the amount of time as set and power itself off. However, if you changed a setting as in you've edited something, this defeats the Power Off setting. It assumes you will want to STORE that edit... Auto Power Off is defeated.

When a sound is *edited* (that is, any change to a stored value is made) a small flag appears to the right of the name, while this appears Power Off is defeated. If, however, you STORE your edit, the flag will disappear, and if no other settings are changed, the countdown to power off will begin... and if the instrument remains untouched it will switch itself off.

There is virtually no danger of the instrument deciding during your gig to power down (unless you've been completely ignoring it). If such is a possibility then set Auto Power = Off. Utility settings are immediately effective and are saved to your Files.

From the Owner's Manual:
"Depending on the instrument status, the power may not turn off automatically, even after the elapse of the specified period of time. Always turn off the power manually when the instrument is not in use."